In recent years, an optical disc, one of the recording media, has been widely used as a means on or from which data, such as video data, audio data, and computer data, are recorded or reproduced. For example, a digital versatile disc (DVD) may be implemented by a multi-layer disc constructed in a stack structure having a plurality of recording layers, from which the reading-out of data is possible, at one side of the disc. A double-layer disc having two recording layers at one side thereof has been practically used as a reproduction-only disc.
In the reproduction-only two-layer DVD, it is possible to read out an electric signal of either a shallow recording layer or a deep recording layer from one side of the disc only by shifting the focal point of a light beam for recording (hereinafter, referred to as ‘focus jumping’). The shallow recording layer is formed of a semitransparent film such that a light beam is transmitted through the shallow recording layer to read out an electrical signal of the deep recording layer. Also, the film thickness of the shallow recording layer and the material for the shallow recording layer are selected. A reflective film is used as the deep recording layer. Between the shallow recording layer and the deep recording layer is disposed an optical transmissive spacer layer exhibiting high transmissivity in a wavelength of the beam for separating the shallow recording layer and the deep recording layer from each other by a predetermined thickness.
In DVD standards, a transparent cover layer on only one recording layer of a single-layer disc, which has the only one recording layer, has a thickness of 600 μm. On the other hand, a two-layer disc is constructed in a structure in which two recording layers are located at depths of 570 μm and 630 μm from an incidence-side surface thereof, respectively, i.e., the two recording layers are arranged above and below the depth, 600 μm, of the recording layer of the single-layer DVD, such that the first recording layer is located above the depth of the recording layer of the single-layer DVD and the second recording layer is located below the depth of the recording layer of the single-layer DVD, while the depth of the recording layer of the single-layer DVD is disposed between the two recording layers. When the two-layer disc is divided into two parts on the basis of the thickness of the single-layer recording layer, as described above, according to the DVD standards, an object lens having a relatively small numerical aperture (NA=0.6), of a pickup optical system for recording and reproducing an electric signal, is designed with respect to a cover layer having a thickness of 600 μm, and a great influence is not exerted on reading out the electrical signal by the difference in depth between the first recording layer and the second recording layer corresponding to approximately ±30 μm from the single-layer recording layer, although the object lens having the small numerical aperture. Consequently, the two-layer disc is adopted. In the related art, as described above, there is adopted a construction to record data on the materials themselves of reflection layers or the materials of the recording layers opposite to the reflection layers.
Meanwhile, high density is further required for the next-generation optical disc. To this end, it is considered to construct a recording layer into a multi-layer structure and, at the same time, to increase the numerical apertures of an object lens. When an object lens having high numerical apertures is used, the amount of aberration generated by an error of the thickness of a cover layer on the recording layer is increased until it becomes impossible to read out an electric signal, with the result that it is not possible to easily reproduce data from the multi-layer recording layer of the next-generation optical disc. For this reason, there is a necessity to incorporate an optical system that is capable of adjusting the amount of spherical aberration into the pickup to perform compensation such that the aberration is not generated in response to the depth of the recording layer.
Also, a conventional optical disc having a multi-layer recording layer structure may be implanted, for example, by a multi-layer optical disc constructed in a structure in which liquid crystal layers, not a semitransparent film, are used, the recording layers and the liquid crystal layers are alternately stacked, and the respective liquid crystal layers are used as optical shutters, thereby selecting the recording layers (See Patent Document 1).
Furthermore, there is known a multi-layer optical disc constructed in a structure in which layer recognition members (for example, formed such that a cholesteric liquid crystal encapsulated in a thin-film type transparent hollow member) are disposed adjacent to the respective recording and reproduction layers of the multi-layer optical disc (See Patent Document 2). Also, pairs of electrodes are disposed such that each layer recognition member is disposed between each pair of electrodes, and intermediate layers are disposed to separate the stacked layers from each other.    Patent Document 1: Japanese Patent Application Publication No. S63-244316    Patent Document 2: Japanese Patent Application Publication No. 2004-79101